The present invention relates to a sealable container particularly, though not exclusively, for the containment and storage of radioactive waste.
So-called intermediate level radioactive waste is generally stored in sealed containers which are initially stored for a period of time, about 100 years for example, in storage vaults. It is intended for these filled and sealed containers to be eventually stored in underground repositories, the repositories themselves being back-filled with a liquid grout to seal the filled containers therein. Alternatively, the repositories may not be back-filled for a further period of about 50 years allowing recovery of the containers if desired.
Present containers are generally rectangular box-shaped and made from stainless steel sheet material having vertical stiffening leg portions situated in a mid-wall position. The corners of the basic box are rounded off to avoid criticality situations arising when four boxes are stacked in a square, corner to corner orientation. However, whilst this is a safe configuration, the presence of the vertical stiffening legs at mid-wall positions reduces the available waste accommodating volume of the container. A further disadvantage of present container designs is that they are constructed by full-thickness penetration welds which are expensive due to the labour intensive preparation needed.
Another form of containment for the storage of intermediate level radioactive waste is by the use of stainless steel drums filled with waste, grouted and sealed and holding four drums for example together in a so-called stillage. Such a stillage for holding waste filled drums is described in WO95/33268 of common ownership herewith. The stillage comprises an open, fabricated sheet metal construction for holding up to four drums, the stillage having four legs at corner positions. The legs are strong in the vertical direction and have features enabling a plurality of filled stillages to be stacked one above the other in stacks of up to nine stillages high for example in a store or repository with no additional interconnection between the stillages. The number of stillages in the stack depends upon the strength of the stillages and, in principle, there could be more than nine in a stack. Such a storage system is described in WO95/33269 also of common ownership herewith. The particular form of construction shown in the above noted prior art references renders the stacked stillages particularly resistant to toppling during a seismic event for example, i.e. the stacked stillages are more inherently stable than known systems of stacked containers.
It is an object of the present invention to provide a sealable container for receiving intermediate level. radioactive waste which container is less expensive to manufacture than known sealable containers.
It is a further object of the present invention to provide a sealable container which is compatible with regard to stacking with the stillages described in WO95/33268 and WO95/33269 without restriction, i.e. there may be any mix of stillages and/or containers adjacent one another and/or in a stack.
According to a first aspect of the present invention, there is provided a sealable container for receiving toxic waste for long-term storage, the container comprising sheet metal members welded together to form a main fillable volume; four hollow, sheet metal, vertically directed leg portions situated at corner positions on the container; and, a separate lid member fixable to the container to seal at least the main fillable volume wherein the majority of the weld length utilised in the container construction is constituted by fillet welds.
Fillet welds are welds formed by the joining together of sheet metal wherein the joint comprises one sheet overlying a second sheet in a parallel or orthogonal relationship for example and a fillet of weld metal is formed between the edge of the first sheet and the surface of the second sheet. In this regard reference is made to British Standards BS 499: Part 1: 1983, the content of which is incorporated herein by reference. Conventional welds for the construction of such containers have employed full-depth penetration welds where two sheet edges abutting each other are ground away to form an included angle of about 90xc2x0 which is then filled with weld metal. Such welds are very expensive to produce owing to the extensive pre-welding joint preparation needed. It has been found that the integrity of containers constructed according to the present invention is adequate for the purpose intended and that the life of such containers is also comparable to prior art containers.
It is intended that the length of fillet welds in the construction of containers according to the present invention should be greater than 50% of the total weld length.
According to a second aspect of the present invention, there is provided a sealable container for receiving toxic waste for long-term storage, the container comprising sheet metal members welded together to form a main fillable volume; four hollow, sheet metal, vertically directed leg members having cavities therein and situated at corner positions on the container; and, a separate lid member fixable to the container to seal at least the main fillable volume wherein the cavities of the four hollow leg members are sealed from the main volume of the container but are fillable from the exterior of the container.
Containers according to the second aspect of the present invention have hollow leg members situated at the container corners. When four containers are stacked adjacent each other in a square array, there is no danger of a criticality situation arising due to too much radioactive material being in close proximity at the corner region since the volume of the leg members is sealed from the main container volume. However, a requirement of containers for long term storage of radioactive waste is that all volumes must be fillable by grout, poured for example into a repository containing a plurality of such containers. Thus, the leg volumes of containers according to the second aspect of the present invention are provided with suitable apertures in appropriate places such that a rising level of liquid grout in a repository, for example, will fill the leg volumes of the containers even when a plurality of such containers are stacked one upon the other and/or stacked with stillages as hereinabove described with reference to the prior art.
Alternatively, the leg volumes may be filled with grout prior to being placed in a long-term storage facility in order to ensure that the leg volumes are entirely filled. In a container where the leg volumes are to be filled prior to storage, the legs will be sealed apart from a suitable aperture at the top thereof for filling purposes.
The construction of the leg portions may take many varied forms. The leg portions may take the form of separate leg member fabrications which are attached to the main fillable volume or may utilise part of a vertical wall member of the main fillable volume to form part of the leg construction.
The fact that the leg members are situated at the container corners is important since the hollow leg members are intended to readily deform if the container is dropped onto a hard surface so as to absorb impact forces. However, the welds at the legs and around the edges of the main fillable volume of the container must remain intact which testing has confirmed to be the case.
According to a third aspect of the present invention, there is provided a sealable container for receiving toxic waste for long-term storage, the container comprising sheet metal members welded together to form a main fillable volume; four hollow, sheet metal, vertically directed leg portions having cavities therein and situated at corner positions on the container; and, a separate lid member fixable to the container to seal at least the main fillable volume wherein a horizontally directed base-wall member of the main fillable volume is provided with a stiffening member on an internal face thereof.
Where containers according to the present invention are to be stacked with other similar containers and/or with stillages holding filled drums as described hereinabove with reference to the prior art, it is necessary that the horizontal base panel member of the container does not sag below a horizontal level and remains at least flat even when filled with waste and grout. In order for the containers according to the present invention and/or stillages according to the prior art when stacked together in stacks of up to nine high, for example, to remain stable, it is essential that the only contact with an adjacent container or stillage is through the ends of the leg members. Thus, sagging of the base panel member in a downwardly direction to contact the container or stillage below is unacceptable due to a dramatic reduction in stability of the stack and damage caused to adjacent items.
Whilst the containers are generally placed upon a flat surface after filling with waste and prior to filling with liquid grout which subsequently cures and solidifies thus becoming self-supporting, in some cases this may not be possible and it is therefore necessary for the container base to be inherently resistant to sagging even when full. Furthermore, the possibility that the containers will not be back-filled with grout and may nevertheless be filled to their maximum load capacity must be allowed for in the construction.
The form of the stiffening member may be of generally cruciform shape extending between opposite walls or between opposite corners of the container within the main fillable volume. The arms of the stiffening member may be of inverted channel shape and welded at least to the container base inner surface. The depth of the cruciform member may be minimised so as not to compromise too much the load capacity of the container with regard to the size of waste which may be contained therein.
Due to the need, in most cases, to preclude any volumes within the container from being unfilled with grout, suitable apertures are formed in the stiffening member to ensure that all spaces are filled by a rising level of liquid grout when grouting the waste filled main volume.
Because the stiffening member is not subject to long-term corrosion attack when the container is filled with grout as is the exterior of the container, it may be made of material other than stainless steel such as a ferritic stainless steel for example. The reason for this is that the grout tends to neutralise any acids formed and to prevent corrosive attack. If the container is not to be grouted, all components may need to be made of austenitic stainless steel.
According to a fourth aspect of the present invention, there is provided a sealable container for receiving toxic waste for long-term storage, the container comprising sheet metal members welded together to form a main fillable volume; four hollow, sheet metal, vertically directed leg portions having cavities therein and situated at corner positions on the container; and, a separate lid member fixable to the container to seal at least the main fillable volume, the container further including a grid to cover the main fillable volume to prevent waste contained therein from rising above the surface of liquid grout until said grout has solidified, the grid, in use, remaining below the surface level of solidified grout.
Some of the toxic waste which will be stored within the container will be of a lower density, perhaps due only to having hollow voids or due to the material per se being of a lower specific gravity than the liquid grout used for filling the container. In these circumstances, it is important to prevent the waste from floating in the grout while still liquid and from protruding above the surface and possibly interfering with lid closure. In the case of radioactive waste, regulations demand that it is completely immersed in grout during storage.
The grid may take the form of a metal mesh having a stiffening frame which fits through the open aperture at the mouth of the container and locks into position at a level within the container which, in use, is below the final level at which solid grout will exist thus, the waste and the grid will be submerged in the solid grout.
Preferably, the grid may be fastened in position in the container by means which are relatively easily operated remotely by manipulator means.